Transmission line installation

ABSTRACT

An apparatus for installing optical fibres (11) in tubes or ducts (29). An optical fibre is fed from a reel (13) into a small diameter feed portion of duct (25), and compressed air is introduced into the duct. This advances the fibre, using viscous drag forces, into a main duct of larger diameter. There is a pressure drop at the intersection of the two ducts. Subsequently, the advancement of the fibre is continued by maintaining the air supply, and the viscous drag forces, acting on the increasing length of the fibre in the duct, advance it along the duct. Two or more fibres, which may be unsheathed primary coated fibres, may be installed simultaneously.

This is a division of application Ser. No. 07/603,711, filed Nov. 21,1990 now U.S. Pat. No. 5,199,689

FIELD OF THE INVENTION

The present invention relates to a method and apparatus for installingtransmission lines in ducts or tubes.

BACKGROUND OF THE INVENTION

European Patent No. 108590 discloses a method and apparatus forinstalling optical fibre members within tubular ducts using fluid dragof a gaseous medium to propel the fibre member along the duct. In theillustrated examples in that patent, a fibre bundle is introduced to theduct via a feed head and wheels, in order to urge the bundle into theduct against a resistive force acting on the advancing end of thebundle.

A number of other methods of introducing fibres into ducts, forsubsequent advancement along the ducts with the aid of viscous dragforces, have since been described.

One particular such method comprises introducing a fibre member to aduct at a point near a high local flow of compressed air, and bleedingsome of the air through a vent some distance, typically around 1 meter,downstream. This method is fully described in our European PatentApplication 0287225A1.

In all practical applications of the fibre blowing process, the natureof the fibre bundle to be installed has been an important factor. Thepresent applicants and others working on the process have investigated avariety of different bundles, in an attempt to develop an optimum rangeof products. One product which has particularly good characteristics isdescribed in our European Patent Application No. 0157610. One of theexamples in that application is a seven-fibre bundle with a tight skinaround the fibres, and an outer sheath of foamed material. Thecharacteristics of this bundle are that it is lightweight, given itsvolume, and its large surface area provides a good surface for theaction of fluid drag forces. There is the further advantage that thetight skin, or inner sheath, locks the fibres together, so that therigidity is increased, and the fibres are unlikely to buckle. Recently,there has been a trend towards using more lightweight bundles with onlya small number of fibres or even a single fibre. Generally for practicalapplications it has been thought important to create a package structureof some kind, usually comprising a foamed layer, to provide a surfacearea to weight ratio which results in the package being blowable overtortuous routes, which may extend over hundreds of meters. Typically,fibre members have been in the range 1 to 3.5 gm⁻¹. Thus, a relativelybulky but light structure was seen as important for good blowingproperties. Additionally, protection for the fibres was seen asessential, because any process carried out on fibres is seen as runningsome risk of damaging the fibres, for example by transmitting excessivetensile or compressive force to the fibres. Thus, robust packaging wasused.

As is well known, the provision of a foamed layer in conjunction withoptical fibres presents a number of practical difficulties, becauseshrinkage of the foam can introduce undesirable compressive forces whichcan adversely affect the transmission properties of the fibres. Theabove-described locking-effect of seven tightly encased fibres is oneway of mitigating this problem. A loose layer of foam is another.Lengthy development has, however, been required to produce a robustproduct suitable for a wide range of working environments.

SUMMARY OF THE INVENTION

The present invention is a modification of the known fibre blowingprocess and method which, most surprisingly, gives excellent results.There are also advantages of cheapness and simplicity.

Thus, the present invention provides a method of advancing atransmission line along a duct with viscous drag of a fluid medium,comprising introducing the transmission line to a feed portion of duct,introducing said fluid medium and advancing the line along the portionand into the main duct, which has a larger diameter than the feedportion, and continuing the supply of fluid to the feed portion of ductduring the installation of the transmission line.

Most surprisingly no venting is required, as in the above mentionedApplication No. 0287225A1. Instead, it is believed that fast flowingfluid in the constricted area of the feed duct acts on the transmissionline in the feed portion of duct, applying drag forces, to introduce itto the main duct, and that a pressure drop, or jet effect, occurs as thefluid expands in the larger diameter main duct. This prevents a build-upof frictional resistance which would otherwise restrict the installationdistance achievable. The feed duct, or jet effect thereby created,apparently acts, in an analogous manner to the drive wheels in FIG. 7 ofEP 0108590, to overcome the so-called hydrostatic force opposing theinsertion of the fibre. Further downstream, the lower drag force actingon the leading part of the fibre in the main duct is sufficient toadvance the fibre along the duct quite rapidly. It should be noted,however, that the precise mechanism is not fully understood and that thecomments above and subsequently on the nature of the process are in nosense to be taken as limiting.

Also very suprisingly, it has been discovered that the process worksvery well in installing primary coated optical fibre, without any outercoating, sheath, or foam layer. Furthermore, several fibres can beinstalled simultaneously into a single duct. Lightweight bundles offibres can also be installed.

The process can accordingly be used in the manufacture of optical fibrecables, in order to introduce fibres into tubes or ducts in the cableswithout risk of damage to the fibre through handling or any kind ofprocessing. Pristine manufactured fibre is simply unreeled and installedwith the aid of compressed air, using the method of the invention, in atube or duct. The drag force acting on the fibre is too low for this torisk damaging the fibre itself. Thus a low cost and highly effectivemethod of introducing fibres into ducts, tubes or the like is provided.

Another particularly useful application is the installation of singlefibres or groups of fibres in ducts in buildings. The ducts themselvesmay typically be robust, polyethylene material, providing effectiveprotection for the fibres after installation, and no intermediate fibrecoating is necessary. It will be appreciated that large cost savings mayresult. This goes completely contrary to previous practice in the art,which has been always to package fibres in a robust manner beforeinstalling them. The packaging, as explained above, has involved muchresearch and development as there are practical difficulties inproviding satisfactory packaging, without damaging the fibre during thepackaging operation or subsequently, or affecting the operatingtemperature range.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example, with referenceto the accompanying FIGS. 1 and 2 which are schematic views of apparatusaccording to an embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIG. 1, primary coated fibre 11 is stored on a reel 13. Thefibre is standard optical fibre, for example a primary coated monomodesilica fibre, having a diameter of around 250 μm, and weighting 0.07g.m⁻¹.

Reel 13 is mounted to be driven by a motor 15, which imparts a verysmall drive force, merely to overcome friction to drive the fibre offthe drum and into chamber 17.

Chamber 17 is formed of a standard plastic `T` piece, having an inletcomprising a hypodermic needle 19 welded into one arm and extendingbeyond an an air inlet 21 provided in the leg of the `T`. Fibre 11enters through needle 19 and is drawn by compressed air supplied atabout 10 bar through inlet 21 into feed tube 25 connected with the otherarm of chamber 17. The fact that the hypodermic needle 19 extends beyondduct 21 means that the fibre is not deflected by the air supply. Feedtube 25 is a narrow tube of about 0.5 mm internal diameter, and length xabout 1 meter. It is attached to connector 23, fixed within the outletarm of the chamber. An outer duct 29 is also connected to chamber 17 bymeans of an outer connector 27. Duct 29 has an outside diameter of 5 mmand an internal diameter of 3.5 mm, and may extend for 200 meters ormore, along any desired route.

In use, the end of fibre 11 is first fed through needle 19, throughchamber 17 and a few centimeters into the feed tube 25. The drive motorand supply of compressed air are both commenced with the air pressureinitially at a low level to avoid problems with back pressure tending topush the fibre back out of the feed duct. A small amount of compressedair leaks from chamber 17 via needle 19, and this helps lubricate entryof the fibre. Due to the small diameter of the needle, the volume of airescaping is slight. Most of the air, therefore, travels down feed tube25 at high velocity, imparting sufficient viscous drag force to propelthe fibre along the feed duct and into the main duct 29. Once the fibrehas advanced into duct 29, the pressure may be increased as desired, egto around 10 bar, to adjust speed of advancement of the fibre to thedesired level. Duct 29 has 3.5 mm inner diameter (5 mm C.D.), and so onentering duct 29, the compressed air expands rapidly and its pressure isreduced. Both friction and drag forces on the fibre are reduced, butviscous drag is still sufficient to advance the fibre along aconsiderable length of duct quite rapidly--for example in oneexperiment, a fibre passed through 275 meters of reeled duct in 51/2minutes.

It will be appreciated from a reading of EP 0108590 that the viscousdrag of air may be used to advance optical fibre members considerabledistances along ducts, eg tens to several hundreds of meters. Theviscous drag force on the fibre members is distributed along the fulllength of the members, thereby enabling the members to be advanced overroutes with many bends. Prior art dragging methods applied high tensionto the fibre member in order to pull them through ducts, and the forceapplied increased dramatically if the route included curves or bends.The method provides a speedy installation method which does not putundue strain on the fibre member, hence the appreciation in thisinvention that primary coated fibres may be installed in this manner.Another characterising feature of the blowing process here and in EP0108590 is that the compressed air, or other fluid medium used toadvance the fibre, has a substantially higher velocity than the fibrethroughout the length of the duct.

The apparatus may easily be adapted to provide for installation ofseveral fibres simultaneously. For example, four fibres may be fed fromtheir respective reels through individual bores or tubes into chamber17, and duct 25. The supply of compressed air advances all four fibrestogether through duct 29.

It will be seen that the apparatus described above differs from that inEP 0287225 in not venting compressed air at any point along theinstallation route. In the illustrated embodiment of our earlierapplication, venting occurred some distance downstream of the air inlet,between about 0.3 to 3.0 meters away, causing a drop in pressure beforethe fibre member passed into the installation passageway. The apparatusdescribed here has a number of advantages, including the fact thatsubstantially all the compressed air supplied is used for advancement(apart from the small fraction leaking through the hypodermic), andthere is therefore greater efficiency in the use of compressed air.

Various changes to the details of the product or process may be made.For example, the configuration of the chamber is unimportant: theresimply needs to be a way of supplying fibre to the feed duct togetherwith compressed air. In the above embodiment, the chamber was providedby a standard `T` piece. This is sufficiently cheap to enable the pieceto be left in situ after installation if desired. It also enables verystraightforward connection and disconnection of the air supply.

Also, the lengths and dimensions of the two portions of duct may vary.The length x of the feed duct may be selected as convenient, eg a fewmillimeters up to several meters, but more usually in the range 50 cm to2.5 meters. Rather than have a feed duct (25) extending within the mainduct (29), the main duct may be connected to the end of the feed ductremote from the chamber. Alternatively a constriction may be formed inthe main duct, fairly close to the entry point of the fibre. Theconstriction, as does the feed duct, forms an area of high flowvelocity, with subsequent expansion of air in a larger diameter region,creating a jet effect. To initiate the process, the fibre is pushed intothe smaller diameter region, or constriction, and the supply ofcompressed air is commenced.

The diameters and relative diameters of the main duct and feed duct maybe varied. Ideally, the ratio of inner diameters is in the range 0.1 to0.8 for feed to main ducts. The feed duct diameter may be as small as0.5 mm as in the above example, or up to around 2.5 mm. The innerdiameter of the main duct may similarly be in the range 3 mm to 8 mm.The various dimensions depend on the desired blowing distance and thenature of the product to be installed (light single fibre or more bulkyfibre bundle, for example).

The inner diameter of duct 25 may be increased if there are severalfibres to be installed, or fibre bundles of some kind, or decreased ifthere is only one or a small number of fibres. The diameter of the mainduct 29 is adjusted accordingly, to preserve the ratio of internaldiameters of the feed to the main duct within a desired range, eg 0.1 to0.8. For instance, when blowing a fibre bundle of 2 mm diameter, a feedduct diameter of 2.5 mm would be suitable. Desirable diameters caneasily be determined by experimentation on the particular combination ofproducts to be used.

As indicated above, one particular advantage of this method is that itmay be used for installing primary coated fibres, which have not beenfurther processed by sheathing. However, the method can be used, ifdesired, to install sheathed fibres or bundles of fibres, or indeed,other kinds of transmission lines, eg copper wire members.

It should be noted that the use of a motor to drive the reel of fibre inthe above example is used as an aid in overcoming the friction of thedrum, and there is no significant force transmitted to the fibre by themotor drive to advance it along the duct, and advancement is achieved bythe viscous drag force of air supplied via inlet 21. A motor may not benecessary if the drum is mounted with low friction bearings.Alternatively, fibre may be torsionally stored on a reel and wound offthe reel, with minimum back tension, simply by the air pressure forcesacting in the duct. Another option is to use loose coils of fibre (egfigure of eight storage).

As indicated above, the method of the invention can be used forinstalling individual fibres, or groups of fibres. For example, two ormore primary coated fibres can be installed simultaneously. Also, lightbundles of several fibres can be installed--for example, the presentapplicants have used a bundle of two fibres in an acrylic coat, providedwith a rip cord which enables the outer coat to be stripped easily toreveal the individual fibres.

The apparatus can easily be connected directly to a break-out assembly.A length of fibre bundle extending from the inlet arm of the chamber(suitably a T-piece as described above) is cut to length, and the ripcord used to strip the outer coat. A tube break-out assembly can than bethreaded over the fibres and plugged into the end of the `T` piece. Aninjection of resin is used to lock the fibres in place. If it is laterdesired, for example, to renew or adjust the fibres, the existing `T`piece and break-out assembly are removed and replaced.

Further, it is possible to blow fibres or fibre members long distancesalong a duct by providing a series of blowing stages spaced somehundreds of meters (eg 500 meters) apart along the duct. FIG. 2 showssuitable apparatus for such serial, or tandem, blowing. At the upstreamend of the duct 29, air inlet (21) is connected to a T-piece 17, asdescribed above. 500 meters downstream, a further T-piece 17' isprovided connected to both the first 500 meter length of duct 29 and thenext 500 meter length 29'. Each T-piece has a length of small diametertubing 25, 25' directed into the downstream portion of duct. At theupstream end of T-piece 17' is a vent 31. The reason for this is that avent has been found necessary to avoid problems with the air flow byallowing air from the first section of duct, 29, to leak away prior toinjection of further compressed air via inlet 21', in order to achievethe required pressure gradient for fibre advancement through duct 29. Itwill be noted that, in contrast to prior art methods, the only vent isprovided immediately upstream of the air inlet--eg within 2 to 50 cm ofair inlet, at each of the second and subsequent blowing stations.

A hypodermic needle 19' is provided downstream of vent 31. This isconnected within `T` piece 17', so as to present an indented conicalsurface to the oncoming fibre, to guide it into the needle 19'.

The air supply to inlet 21' can be connected once the fibre member isobserved to have been carried through the first portion of duct up toT-piece 17'. Alternatively, a sensor (not shown) can be provided todetect the arrival of the fibre member, for automatic switching on ofthe air supply. The sensor may, for example, be a pressure sensor whichdetects a change in flow in the vicinity of T-piece 17'. Provided enoughsources of air supply are available, installation can occurautomatically along many hundreds of meters of duct. Fibre is blownthrough one length after another, by providing a number of blowingstations, each comprising a T-piece, feed tube and source of air supply.As the T-piece and feed tube are inexpensive parts, they can be left insitu after the installation.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

We claim:
 1. Apparatus for advancing a transmission line along a ductwith viscous drag of a fluid medium, comprising an inlet chamberconnectable to a fluid source and a duct with a small diameter feedportion close to the chamber, so that a transmission line introduced tothe feed portion via the chamber can be advanced through the feedportion and introduced into a larger diameter portion of the duct byvirtue of the flow of said fluid through the feed portion from saidchamber into said larger diameter portion, which larger diameter portionof duct is, during advancement of the transmission line, stationary withrespect to the feed portion.
 2. Apparatus for use in situ for advancinga transmission line along a previously-installed duct with viscous dragof a fluid medium, comprising an inlet chamber connectable to a fluidsource, and a duct with a small diameter feed portion close to thechamber so that a transmission line introduced to the feed portion viathe chamber can be advanced through the feed portion and introduced intoa larger diameter portion of the previously-installed duct by virtue ofthe flow of said fluid through said feed portion and without the use ofmechanical drive means to advance the transmission line within the feedportion.
 3. Apparatus as claimed in claim 1, wherein the feed portion isbetween 0.3 and 3.0 meters long.
 4. Apparatus as claimed in claim 1,comprising two or more portions of duct, and one or more intermediatestations between the or each pair of portions, said stations comprisinga chamber connected to the two portions, an air inlet and a smalldiameter feed portion.
 5. Apparatus for advancing a transmission linealong a duct with viscous drag of a fluid medium, comprising an inletchamber and a duct with a small diameter feed portion close to thechamber, the inlet chamber having a transmission line inlet and a fluidinlet for connection to a fluid source, so that the transmission lineintroduced to the feed portion via the chamber can be advanced throughthe feed portion and introduced into a larger diameter portion of theduct by virtue of the flow of said fluid through the feed portion fromsaid chamber into said larger diameter portion, the escape of fluidmedium from said larger diameter portion of duct being precluded by theabsence of effective vents, the larger diameter portion of duct beingstationary with respect to the feed portion during advancement of thetransmission line.
 6. Apparatus for advancing a transmission line alonga duct with viscous drag of a fluid medium, comprising an inlet chamber,the inlet chamber having a fluid inlet for connection to a fluid source,and a transmission line inlet, the transmission line inlet having asmaller cross sectional area than the fluid inlet, and a duct with asmall diameter feed portion close to the chamber, so that a transmissionline introduced to the feed portion through the transmission line inletand via the chamber can be advanced through the feed portion andintroduced into a large diameter portion of the duct by virtue of theflow of said fluid through the feed portion from said chamber into saidlarger diameter portion, wherein, in use, some of the fluid introducedinto said chamber via said fluid inlet escapes from the chamber via thetransmission line inlet.